Significance In this project on newborn (neonate) rhesus macaques followed for the first year of life, data collected on numbers and ratios of lymphoid cell subsets in peripheral blood provided baseline information on immune cell subsets in normal healthy animals. Comparisons of similar data on immune cell subsets can now be made in newborn and infant macaques infected with various viruses including simian immunodeficiency virus (SIV), simian-human immunodeficiency virus chimras (SHIV), rhesus cytomegalovirus (RhCMV), and measles virus. Objectives The objective was to develop baseline parameters for several lymphoid cell subsets in peripheral blood of newborn rhesus macaques by flow cytometry. Seven antibody markers to cell surface differentiation and activation antigens were used in this longitudinal study as follows CD2 (Coulter) for T cells and NK cells; CD20 (Becton-Dickinson) for B cells; CD4 (Ortho) for helper/inducer T cells; CD8 (Becton-Dickinson) for cytotoxic/suppressor T-cells; CD14 (Becton-Dickinson) for monocytes, granullocytes, macrophages, and dendritic cells; CD25 (Becton-Dickinson) for interleukin-2 receptor; HLA-DR (Becton-Dickinson) for B cells, monocytes, macrophages, activated T cells, and NK cells. Results Peripheral blood was collected from six healthy uninfected newborn rhesus macaques at weekly and monthly intervals for the first year of life. Flow cytometry analysis was performed with the seven marker antibodies listed above. These data have been made available to other investigators analyzing the effects of various viruses (SIV, SHIV, RhCMV, measles virus) on immune cell subsets in pathogenesis and vaccine studies. Future Directions New markers are becoming available for characterizing peripheral blood immune cells in rhesus macaques. These markers will allow specific identification of dendritic cells, T naive and T memory cells, monocytes, cell cycle and novel activation markers, etc. KEYWORDS SIV, immune system